In CASCADE control, some systems will request that you select one of three different equations. a)...

In CASCADE control, some systems will request that you select one of three different equations.

a) Explain what is meant by EQUATION A, how it is represented graphically, and where it can be utilized.

b) Explain what is meant by EQUATION B, how it is represented graphically, and where it can be utilized.

c) Explain what is meant by EQUATION C, how it is represented graphically, and where it can be utilized.

Solutions

Expert Solution

a) Equation A :

This is a general purpose calculation to be used if no special reason exists to use another type.

Equation type A is a must for secondary controllers. If eq-type C were to be used in a secondary controller, I-control would be the only control between the OP of the primary controller and the OP of the secondary controller. This would add an unnecessary phase lag of 90 for the primary controller's loop. The result could be an unnecessary destabilization of the primary loop of a cascade control system.

b) Equation B :

The principle considerations, how the control algorithms can work on PV only, are the same as explained for equation type C. Equation type B works as a PI-controller on error (ERR = PV- SP) and works as a D-controller on PV only.

Since eq-type B is in between eq-type A and eq-type C, it’s thus left to the discretion of the user to make decisions about the use of eq-type B. If, E.g : secondary controller needs D-control for stability of the secondary loop and the OP of a primary controller contains all the control actions required for the primary loop (including D-control), then the secondary controller may be best used with eq-type B. In such a case, the full control action of the primary controller is passed on to the OP of the secondary controller via control of the secondary.

c) Equation C :

Equation type C works as I-controller on error (ERR = PV - SP) and works as a PD-controller on PV only.

The mainly used as the ultimate primary controller. An operator cannot cause any sudden control actions that would result in sudden extreme positions of valves and other control equipment. This can only be fully appreciated if one has heard the noise created by the sudden hitting of an extreme valve position of a large valve. It can be felt in almost the whole plant as a big bang. This is most decidedly not good for maximizing the life of a valve.

Manojponduru answered 1 year ago

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